BOWELL : EADUL^ OF BRITISH HELtCIDS. 381 



examine the rest of the anatomy of Z. Algirus^ we find that it is quite 

 of the true Vitrea type, except for such modifications as are evidently 

 the result of its hypertrophic growth. All our Helicidae, with the 

 exception of the Vallonias, present the character of many admedians, 

 but especially is this the case with the larger specimens and larger 

 individuals ; and this holds good with regard to the Succine(s also. 

 The fact assumes a special importance when we observe that it is 

 an example of change in size inducing definite type of morphological 

 change, and doing so for a mechanical reason which it is not hard 

 to understand. As the causes of change in size may be traced back to 

 the action of cosmic forces, acting rliythmically during the whole 

 of the biological period, this definite change of symmetry which results 

 from change of size may be supposed to form an important factor in 

 evolution. 



Let us next ask ourselves what is the significance of the marginals. 

 It is possible to tell at a glance in most cases whether we are dealing 

 with a Limacid, or a Suceinea, or a Helicid, by looking at the 

 marginals ; and, as has already been mentioned, there are two Helicid 

 types, one with bifid mesocone (like the Helices), and the other with 

 simple mesocone (like the Arions). What is the relationship between 

 these two forms ? The latter is illustrated in the first six of the species 

 described in this paper. 



Measurements, additional characters, and diagnostic notes are here 

 given according to the plan adopted in Part II. 



ADDITIONAL CHARACTERS. 



[The first description in each case is that of the centrals, the 

 second of the admedians, and the third of the externals.] 



(«) Shape of basal plate. 



virgata. Rounded oblong, larger below ; not markedly alate ; 



upright oblong, upper margin irregular. 

 cantiana. With indentations above and below; alate; subtetragonal. 

 revelata. Roughly hexagonal; modified central ; depressed oblong. 

 sericea. Upright oblong ; as central ; squared. 

 hispida. As sericea, but more curved. (See note below.) 

 rufescens. As hispida, but generally more elongated. 

 caperata. As virgata, but externals small, irregular. 

 Itala. Roughly hexagonal ; rounded oblong; irregular triangulate. 

 harhara. Key-shaped, doubly notched below ; first oblongate, then 



alate ; depressed oblong. 

 graymlata. Rounded oblong, larger below ; rhomboid ; depressed 



oblong. 

 ih) Character of lower corners of basal plate. 

 virgata. Rounded ; inclined ; squared. 

 cantiana. As virgata, but with characters intensified. 

 revelata. All considerably shortened. 

 sericea. Angles not prominent. 

 hispida. Approaching vinjala type. 



